1. Field of the Invention
The present invention relates to a scan type image recording apparatus in which images are produced on a recording medium by a scanning light beam in accordance with information signals.
2. Description of the Prior Art
Recording apparatus has been developed and is known in the art in which a beam of light is modulated in accordance with image information coming from an electronic computer or the like. The modulated beam is focused on a recording medium while scanning through optical elements such as a light deflecting device and a lens to effect recording of the image information.
The known image recording apparatus mentioned above has a problem which is attributable to the type of developing process used in the invention. For example, two developing processes may be compared. One is of the type in which those portions not exposed to the light beam are visualized and the other is of the type in which exposed portions are visualized. Compared with the latter mentioned developing process, the former has a problem. Namely, when the first mentioned type of developing process is employed, there occurs a phenomenon of so-called "thinning of image" (line becomes thinned or narrowed in the direction of beam scanning). This phenomenon of "thinning of image" will be described in detail with reference to FIGS. 1 to 4.
FIG. 1 shows a letter "T" formed by employing the second developing process (exposed part is developed). The width of line in the scanning direction is indicated by 101 at the beam portion of T and by 102 at the leg portion. In contrast, when the first mentioned developing process (unexposed part is developed) is employed, a T as shown in FIG. 2 is formed which has a line width of 201 at the beam portion and 202 at the leg portion. It is clearly seen that the widths, 201 and 202 are smaller than 101 and 102 respectively. This is the phenomenon of "thinning of image". Due to this unfavorable phenomenon, the image "T" obtained employing the first mentioned type of developing process is not good in quality as compared with that obtained by the second mentioned one.
The cause for this unfavorable phenomenon will be described in further detail with reference to FIG. 3.
In FIG. 3, a scanning line segment is analytically shown. When the second mentioned developing system (exposed part is developed) is employed, at the rising point 310 of an image (video) signal 300 there is produced a scanning spot at location 312 on the recording medium. The scanning spot continues running in the scanning direction indicated by arrow 314 during the time that the signal is On. At the time point 311 when the signal becomes Off, the scanning spot not at location 313 disappears. 315 designates the center line of a scanning line segment thus formed. The distribution of exposure along the center line 315 is shown at 302. At the area from one vertical edge 316 to the other vertical edge 318 of the scanning spot at location 312, the amount of exposure increases gradually. Then, the exposure continues to be constant until the spot enters the area between one vertical edge 319 and the other vertical edge 321 of the scanning spot at location 313. At this area between 319 and 321, the amount of exposure decreases gradually. The level of exposure at the centers 317 and 320 of the scanning spot location 312 and 313 is a half of a peak constant level. For this distribution of exposure 302, the exposed part above the developing level 322 is developed and other exposed parts under the developing level 322 can not be developed according to this type of developing process. Therefore, in this case the width of developed image is 323 which corresponds to the line width 101, 102 in FIG. 1.
When the first mentioned developing process (unexposed part is developed) is employed, images are formed by means of image signal 304 which is an inverted signal of the above-mentioned image signal 300. At the time point 350 when the image signal 304 becomes Off, the scanning spot reaches the spot location 352 and disappears. At the time pont 351, the signal becomes On and a scanning spot at location 353 is produced which runs in the scanning direction indicated by arrow 354. The scanning line section thus formed has its center line at 355. The distribution of exposure along the center line 355 is shown at 306. The amount of exposure decreases gradually at the area between one vertical edge 356 and the other vertical edge 358 of the scanning spot location 352. At 358, the exposure becomes zero and thereafter it continues to be zero until 359. At the area between one vertical edge 359 and the other vertical edge 361 of the scanning spot 353, the amount of exposure increases gradually and thereafter it is kept constant. The exposure level at the centers 357 and 360 of the scanning spot location 352 and 353 is half of the constant level. Let 362 be the developing level of the first mentioned type of developing process, then the exposed part under the developing level 362 and all of the unexposed area are developed. The remaining part can not be developed. Therefore, the width of the developed image becomes that indicated by 363 which corresponds to line width 201, 202 shown in FIG. 2.
For the reason described above, when the developing process of the type by which unexposed an part is developed is employed, the width of development becomes 363 in FIG. 3 (or 201, 202 in FIG. 2) which is shorter than 323 in FIG. 3 (or 101,102 in FIG. 1), namely the width of development by the second mentioned type of developing process (exposed part is developed). Thus, the phenomenon of "thinning of image" is caused. The image quality of "T" in FIG. 2 is reduced when compared with that in FIG. 1 accordingly.
The degree of "thinning of image" varies case by case. At the time of background exposure, when the case where unexposed portions concentrate in an area is compared with the case where an unexposed part stands isolated, these two cases represent different degrees of "thinning of image". Also, the degree of "thinning of image" varies depending upon the type of developing system then used. This will be described in detail with reference to FIG. 4.
FIG. 4A shows an original image or an image pattern to be recorded. The black part 400 comprises an isolated black portion 400a and densely concentrated black portions 400b. The width of each black portion is denoted by Wd and the width of white portion between two black portions 400b is denoted by We. In this connection, it should be understood that both of the widths Wd and We are in the order of 0.1 mm.
By reading the original image shown in FIG. 4A along the line indicated by arrow 403 using an image pick-up element, there can be obtained an electric signal 404 as shown in FIG. 4B. This electric signal 404 is used as a signal for modulating a laser beam. 405 indicates the laser beam incident upon a photosensitive medium. The abscissa in FIGS. 4C, D, E and F indicates the positions on the photosensitive medium in the principal scanning direction of the laser beam. According to the laser beam 405 there is formed on the photosensitive medium an electrostatic latent image as shown in FIG. 4D.
If the electrostatic latent image is developed by a developing agent of a dry two-component toner system composed of resin toner and iron powder carrier and using a magnet brush having no edge effect, then there is produced a developed image as shown in FIG. 4E. In this case, the thinning of image is uniform throughout the isolated black portion and the closely spaced black portions.
On the contrary, if the latent image is developed by using a single component toner projection developing system as disclosed in U.S. patent application Ser. No. 06/058,434 filed on July 18, 1979, then there is formed a developed image as shown in FIG. 4F. The degree of thinning of image of the isolated portion, in this case, is larger than that of the closely spaced black portions. This is attributable to the fact that in the toner projection development, the electric force lines at the edge port of a latent image are not directed to the developing electrode from the latent image, which leads to the phenomenon of thinning of image. At the inner side portion of the closely spaced black portions, this phenomenon can be lessened. However, at the isolated black portion, it becomes pronounced.
In summary, when the toner projection development is employed, the larger the white background area is, the larger the degree of thinning of image. The degree of the phenomenon "thinning of image" is therefore reduced with decrease of the area of the background.